Question about Tamron 17-50mm 2.8 front focussing in low light

Hi,

just got a Tamron 17-50mm 2.8 lens, and I used the focus test chart from http://focustestchart.​com to make sure it is a good copy.
I noticed that when I follow instructions I get a front focus of about 20mm according to the chart (See picture below). This is with regular indoor light not specifically pointed to the chart. I reproduced this with multiple shots in different appertures on two days, redoing the whole setup (tripod and all).

However, when I specifically point halogen lights onto the chart to create a more perfect lighting situation, I get reproducably near-perfect focus.

I tried the same thing with my 50mm 1.8 lens, and it was right on target all the time.

Question: What do you think is the case here?

a.) I got a bad copy of the Tamron lens (=> I get another copy if it bothers me)
b.) This is a limitation of the Tamron 17-50 2.8 (=> I get another lens type if it bothers me)
c.) I did something wrong in the setup, the above doesn't make sense or is not conclusive. (=> The lens is good, I'm not

Thanks for your help,
Zak

Are you using centre focus point only? If not, then do so and try the test again.

Welcome by the way ... I like the logic of your alternative solutions

I'm doing center focus point smack in the middle of the "Focus here" black line. Good point, though.

EOS cameras require contrast to focus. Dim light means less contrast. Putting light directly on the subject increases that contrast. Lenses without USM tend to need more contrast than those with (variable from lens to lens though and with exceptions). Your Tamron does not have USM.

Ok, well if you are getting smack in the centre with decent light it seems the problem is simply light isn't decent enough to get a proper focus. You reckon? How about taking some proper shots of real world stuff in normal light and then looking at them to see if there is still a problem.

My Tamron 17-50 seems to be acting the same way as yours, at first I thought it was a problem with my camera / lens combination but after a few more test shots, I couldnt reproduce the front focus issue, (this maybe because I moved to another room, which has more ambient light, now that I think about it)

I'd say stop measurbating and start shooting.

A. How certain are you that the focus point was not reading the line in front of 'focus here' line (the zone is bigger than the little diamond that lights up)
B. That chart is not as good as a real scene, like focusing on a series of books that are on the shelf, staggered back from one another.
C. You are really gonna worry about 20mm?!?!?!

My situation is the following: After careful research I found the Tamron the best fit for my needs/budget combination: Making pictures of our baby in sub-optimal lighting conditions without having to change lenses much, budget about $500. I like the Canon 17-55mm 2.8, but it's way out of budget. On the other hand, I want to see a difference from the kit lens for spending 4x as much.

I'm not an experienced photographer, so I trust the lots and lots of positive reviews that the Tamron 17-50 lens is good. But I've also read a bunch of reviews talking about production quality issues and bad lenses. I want to make sure I don't have one of them.

I've made a good amount of normal pictures, some of them super sharp, a fair amount of them ok-sharp, some out-of-focus. I cannot judge if this is because of my limited abilities or the lens, so I'm looking for an objective focus quality test.

Wilt wrote in post #2128587
A. How certain are you that the focus point was not reading the line in front of 'focus here' line (the zone is bigger than the little diamond that lights up)

This is a good point. Do you have information how big the center zone is for a Canon 400d/XTi? I assume that this focus chart was made to prevent this issue, as there is a lot of unprinted space above and below the "Focus here".

condyk wrote in post #2127953
Ok, well if you are getting smack in the centre with decent light it seems the problem is simply light isn't decent enough to get a proper focus. You reckon? How about taking some proper shots of real world stuff in normal light and then looking at them to see if there is still a problem.

Just for reference, the light for the picture above was normal room light, the camera took the the above picture as 1/160 sec with F/3.5 at ISO400 and 50mm. I'm afraid my normal indoor pictures will rarely get light that good. In the house with lights on, most of my shots measure at 1/25 to 1/50 with F/2.8 and ISO 400.